1. Field of Invention
The present invention relates to a method and system to process a set of tomosynthesis slices, and to a computer program product to implement the method.
It applies in particular, but is not limited to, a 3D radiography imaging technique called DBT (“Digital Breast Tomosynthesis”), or to so-called conventional 2D radiography of the breast or other organs.
2. Description of the Prior Art
Tomosynthesis imaging is a known technique in three-dimensional imaging.
With reference to FIG. 1, an image acquisition machine for tomosynthesis generally comprises an X-ray emitter 12 and a detector 11.
An object of interest O, for which it is desired to reconstruct a three-dimensional volume in the form of a set of slices, is arranged in the vicinity of the detector 11, on a platform 16 parallel to the detector for example.
The emitter 12 can be placed at different positions, which allows X-rays to be emitted in the direction of the object of interest at several angles. The X-rays are detected by the detector 11 after passing through the object; several images in two dimensions are thereby acquired from several angles of projection.
The angles of emission are limited, for example to ±15° relative to the vertical to the object.
These images are then processed to obtain a set of slices representing a three-dimensional digital reconstruction of the object.
In this set of slices, a practitioner seeks to identify lesions, any sites of micro-calcification or opaqueness for example in a breast, or any nodules in the lungs which are potentially cancerous. The practitioner can also seek to identify a bone fracture e.g. of the hand or shoulder. These lesions and fractures which are visible using a radiography imaging technique can generally be termed as “radiological signs”.
In the state of the art of breast cancer screening, the slices are generally separated by a thickness of around 1 mm for a mean total of around 50 to 70 slices to be examined by the practitioner.
The size of microcalcifications varies approximately between 100 μm and 1 mm. Owing to the limited emission angles, these microcalcifications artificially appear in elongate form in the digital reconstruction of the breast, along an axis perpendicular to the detector. On this account, slices separated by a thickness of 1 mm are generally sufficient for their detection.
The shape of a microcalcification, however, is much better revealed by a slice passing through the object itself rather than by its reconstruction artifact.
It is also known that the Contrast-to-Noise Ratio (CNR) of a spherical microcalcification is optimal when a slice passes through the centre of the microcalcification. When diagnosing, the practitioner will pay particular attention to the shape of microcalcifications, an irregular shape possibly being a sign of malignity.
It would therefore be of interest to increase the probability of obtaining slices passing through microcalcifications to allow the practitioner to characterize these calcifications more easily, for example by reducing the sampling interval of the digital reconstruction.
The space between the slices would then be narrower—for example 0.5 mm rather than 1 mm—and the number of slices in the set would be increased—for example doubled which is not without raising problems.
First, the practitioner would have more slices to analyze, which implies more time-consuming examination and increased fatigue through repetitive examinations, which may generate errors of inattention and, in extreme cases, a wrong examination result which is contrary to the targeted objective.
Additionally, the quantity of data to be stored would increase with the number of slices, which raises non-negligible problems of memory space.